Growth factor receptor/tyrosine kinases of the ErbB family regulate the growth, differentiation, fate and survival of cells in a variety of tissues, and play critical roles in cardiac and neural development. The aberrant activation of these receptors is also believed to be involved in the genesis and progression of breast and ovarian tumors. The ErbB receptors are activated by members of the EGF family of growth factors. Numerous studies suggest that these receptors undergo ligand-stimulated homo- and heterodimerization events that play key roles in mediating receptor tyrosine phosphorylation and the initiation of downstream signaling cascades. The ultimate objective of this project is to understand the involvement of ErbB receptor-receptor interactions and phosphorylation in mediating cellular responses to EGF-like growth factors. The hypothesis guiding this study is that the different EGF- like growth factors stimulate the heterodimerization of different pairs of ErbB receptors, and ligand-stimulated receptor phosphorylation then triggers intracellular signaling cascades that lead to specific cellular responses. Differential receptor phosphorylation through receptor heterodimerization may underlie the range of responses of a single cell to the different EGF-like ligands, or the range of responses of different cell types to a single ligand. A detailed analysis of the hierarchy of ligand-stimulated homo- and heterodimerization events among the four known ErbB receptor family members will be carried out using the Sf9 insect cell expression system together with three assays for receptor-receptor interactions. Recent observations that the ErbB3 receptor lacks an intrinsic tyrosine kinase activity call into question the allosteric oligomerization model for transmission of a signal across the plasma membrane by ErbB receptors. This model, as well as three alternative hypotheses, will be critically tested using purified inset cell-expressed ErbB2 and ErbB3 receptors (and kinase-inactive mutants) reconstituted into phospholipid vesicles. In addition to the suggestion that the different EGF-like ligands preferentially induce the heterodimerization of different pairs of ErbB receptors, very recent studies suggest that the specific residues that become phosphorylated within a particular ErbB receptor in response to growth factor binding depends on the identity of the EGF-like ligand presented to the cell. The role of differential receptor phosphorylation in mediating cellular responses will be determined by examining the effects of two EGF-like ligands, NRG1 and NRG2, on the phosphorylation of ErbB receptors in proliferating and differentiating cells.